1. Field of the Invention
This invention relates to focusing screen devices suited for use in optical instruments which necessitate focus adjusting operation, such as photographic cameras, and more particularly to focusing screen devices which enable the observer to recognize the focus adjusting condition of the optical system by discrimination of whether or not the image is shifted at least in part.
2. Description of the Prior Art
Conventional focusing screens, such as those used in single lens reflex cameras, often use matted surfaces, microprisms, or split image prisms. Matted surfaces and microprisms have the disadvantage that the user must be experiened in discriminating between a sharp focus condition and an out-of-focus condition because the sharpness of the image formed thereon must be observed directly. On the other hand, a split-image prism does not require as much experience and focusing is comparatively simple. However, because the image is split at only one line segment when the image is out-of-focus, the operator can often not use it. To ensure proper operation, microprisms and split-image prisms are often incorporated into a single focusing screen. Such a focusing screen requires proper selection of one of the two possible focusing methods depending upon the given photographic situation. Thus, it is rather difficult for a beginner to master the technique of focusing with such a screen.
Moreover, there is a device in which two optical band grooves of saw-tooth-like or peak-like cross-section are arranged to face in different directions and to be mechanically reciprocatingly movable so that the field of the view of the finder is covered by either one of the band grooves at a time. Accordingly, as the two band grooves reciprocate, when the image is not in-focus, the image shifts to the left and right periodically. When a sharp focus condition is reached, the image is at a standstill. This method of achieving detection of an in-focus condition is disclosed in Japanese Patent Publication No. Sho 46-33496 (33496/1971). A device which makes it possible to detect when the image is in-focus by rotating the saw-tooth-like or peak-like cross-sectional grooves mechanically when the image is out-of-focus, and producing a standstill when the image is in-focus, is disclosed in Japanese Patent Publication No. Sho 47-20733 (20733/1972). These devices permit an operator to focus clearly and detect a proper focus simply even if the operator is a beginner.
However, because these devices require mechanical drive means to move the optical band grooves of the focusing screen, and, further, because the focusing screen travels through a large space, it is very difficult, if not nearly impossible, to incorporate such a device in a space as small as, for example, that available within a camera housing. Even if such incorporation is possible, as the focusing screen of the band grooves is driven, vibrations are produced. This raises the problem of the possibility that the focusing screen plane cannot be kept sufficiently stable to meet the particularly rigorous requirement for accuracy.